ctutils: constant-time selection and equality testing #11

Job	Run time
test (1.87.0)	19s
test (stable)	14s
build (1.87.0, wasm32-unknown-unknown)	18s
build (stable, wasm32-unknown-unknown)	12s
test-careful	57s
build (stable, thumbv7em-none-eabi)	12s
build (1.87.0, thumbv7em-none-eabi)	19s
minimal-versions / minimal-versions	20s
	2m 51s

Provide feedback